There is only one rule: There are no rules. That's entertainment. Everyone is an opportunist to some degree, and if you are the guy trying to sell a Banksy, you might as well make a documentary. And if you want to intellectualize it ""HOW TO SELL A BANKSY" raises questions of ownership, authentication and the true value of art itself," says the filmmakers. We just like trailers, because we don't need to see the movie now. Watch it after the jump . . .